im pretty confident in getting 10 more mows out of all my decent customers and im pretty quick to hit the drop button on less than the best as well, which would be another drama.
Like colsy said, it shifts the relationship. i reckon if people have pre paid they own you a bit and if you cant fit them in one time or another you can end up on tracy grimshaws show.
i had a referral offer when i started, it sucked though because it appealed to tight ar5es and you end up getting referrals to other tight ar5es and people miles away. Good customers are happy to refer a good service to their friends to help them, rather than for their own gain.